A packaging material for foods, beverages and the like is demanded to have such functions as strength, resistance to cracking, heat resistance and the like for protecting the content from distribution, storage, such as refrigeration and the like, and treatments, such as thermal sterilization and the like, and also demanded to have a wide variety of functions, such as excellent transparency and the like for confirming the content. In recent years, furthermore, it is demanded to have oxygen barrier property for preventing oxygen from invading from the exterior for suppressing foods from being oxidized, carbon dioxide barrier property and barrier property against various kinds of aromatic components and the like.
A polyolefin, such as polyethylene, polypropylene and the like, a polyester, such as polyethylene terephthalate, and an aliphatic polyamide, such as nylon 6, have been used widely as a packaging material owing to good handleability and processability, and excellent transparency and mechanical properties of a sheet or film obtained therefrom. However, it is poor in barrier property against a gaseous substance, such as oxygen and the like, and thus has such problems that the content is liable to suffer oxidation degradation, and aromatic components and carbon dioxide are liable to penetrate therethrough to shorten the expiration period of the content.
A plastic container (such as a bottle) containing mainly a polyester, such as polyethylene terephthalate (PET) and the like, has been widely used for tea, fruit beverages, carbonated beverages and the like. The proportion of a small-size plastic bottle within the plastic containers is being increased year by year. The ratio of the surface area per unit volume of the content is increased by decreasing the size of the bottle, and therefore, there is a tendency of shortening the expiration period of the content when the size of the bottle is decreased. In recent years, furthermore, beer, which is liable to receive effects of oxygen and light, is sold in a plastic bottle, and tea in a plastic bottle is sold in a warmed state, which expand the application range of plastic containers. Under the circumstances, there is a demand of further enhancing the gas barrier property of a plastic container.
For enhancing the barrier property against a gaseous substance, such as oxygen, a film or the like formed by combining the aforementioned thermoplastic resins with a gas barrier resin, such as vinylidene chloride, an ethylene-vinyl alcohol copolymer, polyvinyl alcohol and the like, is used. However, a film containing vinylidene chloride is excellent in gas barrier property irrespective of the storage conditions, but has such a problem that it generates dioxin through combustion to contaminate the environments. An ethylene-vinyl alcohol copolymer and polyvinyl alcohol are free of the before-mentioned problem of environmental contamination. However, a film formed therefrom exhibits excellent gas barrier property under an environment of a relatively low temperature, but there is a tendency of decreasing the gas barrier property considerably in the case where the content to be kept has a high water activity or is kept under a high humidity environment, or the film is subjected to a thermal sterilization treatment after charging the content, whereby there may be a problem in storage stability of the content.
As a material excellent in gas barrier property, a polyamide containing a m-xylylene group in the main chain thereof obtained through polycondensation reaction of m-xylylenediamine and an aliphatic dicarboxylic acid, such as polyamide MXD6 obtained from m-xylylenediamine and adipic acid, has been known. The m-xylylenediamine group-containing polyamide is used widely as engineering plastics owing to the high rigidity and excellent thermal property and molding processability. It exhibits high barrier property against a gaseous substance, such as oxygen or carbon dioxide gas and the like, owing to the m-xylylene group in the polymer main chain, and is used as a gas barrier material for various packaging materials, such as a film, a bottle, a sheet and the like, by combining with various resins, such as polyethylene terephthalate and the like. However, upon molding the m-xylylene group-containing polyamide into a film, a sheet, a bottle and the like, air may be entrained upon melt processing to form bubbles, or appearance failure, such as silver stripes, uneven flows and the like, may occur unless the molding conditions, such as the screw shape, the temperature, the back pressure and the like, are properly set. In the case where a large amount of powder is contained in the pellets, particularly, there is a tendency that these phenomena may often occur, and there is a demand of improvement.
For preventing entrainment of air from occurring in molding process, in general, it has been necessary to provide such measures that the lower side of the hopper is cooled to decrease the temperature of the cylinder of the extruder, the rotation number of the screw is lowered, the back pressure is increased in the case of injection molding, and the like (see Non-patent Document 1), but even though these measures are made, there are cases where entrainment of air cannot be sufficiently prevented from occurring due to the defective shape of the screw, and there is a problem of deterioration in yield of products.
A polyamide resin composition excellent in extrusion property that suffers less unevenness in ejection and can lowers the extrusion force upon melt extrusion molding has been disclosed (see Patent Document 1). Although the technique can improve the extrusion property upon extrusion molding, there are cases where entrainment of air cannot be prevented from occurring depending on the shape of the screw. Furthermore, there is no study relating to injection molding.
Since a m-xylylene group-containing polyamide has crystallinity, there are cases where it is whitened due to crystallization immediately after molding to lower the transparency of the resulting product unless molding conditions such as an extrusion temperature, a cooling temperature and a cooling time are properly set. For preventing the whitening due to crystallization immediately after molding, a certain degree of improvement can be obtained by decreasing the cooling temperature or prolonging the cooling time, but the measures provide a problem of deterioration in economy due to the prolonged cycle time. Furthermore, in such an apparatus that the cooling temperature cannot be sufficiently lowered, or the cooling time cannot be prolonged, due to the specification of the apparatus, the m-xylylene group-containing polyamide cannot be used.
A polyamide molded article containing solid phase polymerization polyamide MXD6 that suffers less whitening upon storing under a high humidity, upon making into contact with water or boiling water, or upon heating to a temperature of the glass transition temperature or higher is disclosed (see Patent Document 2), but there is no study relating to prevention of crystallization immediately after molding, and a polyamide that is not formed by solid phase polymerization.
In a m-xylylene group-containing polyamide, carbon at the α-position of the benzene ring (benzyl carbon) is liable to be a radical, and thus it is poor in thermal stability as compared to a polyamide, such as nylon 6. Accordingly, various proposals have been made relating to improvement in thermal stability upon production or extrusion molding process.
For example, for producing a m-xylylene group-containing polyamide having a less amount of gel, it is important to execute polycondensation in such a manner that the intended molecular weight is quickly obtained while reducing the thermal history as much as possible. For reducing the thermal history, it is effective to execute the amidation reaction quickly by adding a compound having a catalytic effect into the polycondensation system.
As the compound that catalyzes the amidation reaction, a phosphorus atom-containing compound has been widely known. Such a method has been previously proposed that polycondensation for producing a polyamide is carried out in the presence of a phosphorus atom-containing compound and an alkali metal compound (see, for example, Patent Document 3). The phosphorus atom-containing compound not only accelerates the amidation reaction, but also functions as an antioxidant that prevents coloration of the polyamide due to oxygen present in the polycondensation system, and therefore a polyamide having a less amount of gel and having a low yellowness degree can be obtained. However, the compound may bring about formation of a three-dimensional structure (gelation) in some cases, and thus a suitable addition amount is necessarily selected.
In the case where a polyamide having a phosphorus atom-containing compound added in the polycondensation step is remelted and molded in an extruder or the like, however, there are cases where the resin pressure is gradually increased to fail to execute stable operation. As a result of research on the reasons therefor by the inventors, it has been found that the phosphorus atom-containing compound contained in the polyamide at the filter mounted on the discharge port of the extruder is denatured and deposited to clog the filter by attaching thereto.
Such a method has been proposed to prevent clogging of the filter by decreasing the addition amount of the phosphorus atom-containing compound, the alkali metal compound and the like added to the polyamide (see, for example, Patent Document 4). However, this method is different from the present invention, which pays attention to denaturation of the phosphorus atom-containing compound. In this method, furthermore, since the addition amount of the phosphorus atom-containing compound for preventing coloration of the polyamide is small, the resulting polyamide is colored yellow and has a low utility value as a packaging material.
A method for preventing gelation of a polyamide by adding from 0.0005 to 0.5 part by weight of at least one kind selected from a lubricant, an organic phosphorus stabilizer, a hindered phenol compound and a hindered amine compound upon molding a polyamide has been proposed (see, for example, Patent Document 5). However, this method relates to prevention of gelation of the polyamide due to the thermal history during molding process, but there is no disclosure relating to clogging of the filter ascribable to denaturation of the phosphorus atom-containing compound in the polyamide.
A m-xylylene group-containing polyamide has various problems upon applying as it is to a purpose that requires flexibility, such as a film and the like, due to the considerably high rigidity thereof. For improving the property, various proposals for satisfying both the gas barrier property and the flexibility have been made by melt-mixing an ordinary polyamide excellent in flexibility, such as nylon 6, nylon 666 and the like, with the m-xylylene group-containing polyamide, or by forming a multi-layer structure therewith (see, for example, Patent Documents 6 to 8).
However, when the m-xylylene group-containing polyamide is mixed with another nylon, there are cases where the melt viscosity is increased far beyond the value that is expected from the arithmetic average. As a measure for preventing the phenomenon, it has been proposed that the difference in concentration between the end carboxyl group and the end amino group in the polyamide resin composition after melt-mixing has a particular relationship to the concentration of the phosphorus atom contained in the polyamide resin composition after melt-mixing (see, for example, Patent Document 9). In this method, for suppressing the amidation from proceeding in the molten state, the balance of the end groups of the polyamide is set to make one of them excessive, or the amount of the phosphorus compound capable of functioning as an amidation catalyst is decreased, whereby the increase in melt viscosity due to increase of the molecular weight is prevented from occurring. Upon production of a polyamide used for a packaging material and the like, however, a sufficient polymerization degree cannot be obtained unless the reaction molar ratio between the diamine component and the dicarboxylic acid is made close to 1 as much as possible. Accordingly, in this method, it is practically necessary to decrease the concentration of the phosphorus atom in the polyamide to a low level. In the case where the concentration of the phosphorus atom in the polymerization system is low, the polymerization reaction time for providing a sufficient molecular weight is prolonged, and the resulting polyamide has an increased yellowness degree and contains a large amount of gel due to oxidation of the polymer, whereby the products obtained by the method, such as a packaging material and the like, are poor in commercial value consequently.
The inventors have found that the melt viscosity of the melt-mixed product of the m-xylylene group-containing polyamide and another nylon varies in some cases depending on the production history, the storage condition, the storage period and the like of the m-xylylene group-containing polyamide even though the melt viscosities, the phosphorus atom concentrations, the end group concentrations and the like of the starting materials are the same. The problem, which is not disclosed not only in Patent Document 9 but also in any document, is necessarily resolved from the standpoint of extrusion process stability.
[Patent Document 1] JP-A-10-147711
[Patent Document 2] JP-A-2000-248176
[Patent Document 3] JP-A-49-45960
[Patent Document 4] JP-A-2005-194328
[Patent Document 5] JP-A-2001-164109
[Patent Document 6] JP-A-11-334006
[Patent Document 7] JP-A-2000-211665
[Patent Document 8] JP-A-2003-011307
[Patent Document 9] JP-A-7-247422
[Non-patent Document 1] “Shiritai Shashutsu Seikei” (Learning Injection Molding), published by Japan Machinist Co., Ltd.